A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay (the accuracy of alignment between patterns formed in different patterning steps, for example between two layers in a device) and defocus of the lithographic apparatus. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
Methods and apparatus for determining structure parameters are, for example, disclosed in WO 20120126718. Methods and scatterometers are also disclosed in US20110027704A1, US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates an illumination spot that is smaller than the grating (i.e., the grating is underfilled). In addition to scatterometry to determine parameters of a structure made in one patterning step, the methods and apparatus can be applied to perform diffraction-based overlay measurements.
Diffraction-based overlay metrology using dark-field image detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple targets can be measured in one image. Examples of dark-field imaging metrology can be found in international patent applications US2010328655 A1 and US2011069292 A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US20120044470A US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. Similar techniques have been developed for measurement of focus and dose using modified small targets. Methods of determining dose and focus of a lithographic apparatus are disclosed in documents WO2014082938 A1 and US2014/0139814A1, respectively. The contents of all the mentioned applications are also incorporated herein by reference.
Therefore, in known intensity-based scatterometers parameters of interest such as overlay, CD and focus are inferred by measuring the intensity of radiation diffracted by appropriate targets. For example, in diffraction-based metrology using dark-field imaging, results are obtained by measuring the target in such a way as to obtain separately the −1st and the +1st diffraction order intensities. Comparing these intensities for a given grating provides a measurement of asymmetry in the target. The measured asymmetry can then be converted to a measurement of overlay, focus or dose, depending on the form of the target, which is specifically designed to have an asymmetry that is sensitive to the parameter of interest.
The known examples and methods measure only the intensity of scattered radiation using incoherent light sources. Reconstruction of the target is an ill-posed inverse problem which cannot be solved without prior information about the target. To solve the ill-posed inverse problem when using current inspection apparatuses, relatively large target structures are required for the extraction of parameters of interest. Similarly, the known dark-field imaging metrology with small targets measures only the intensity of different diffraction orders, and measurements are undesirably sensitive to process-induced variations. That is to say, the measurement does not distinguish between asymmetry due to the parameter of interest and asymmetry or other variations caused by process variations.
In US2012243004A1 it is proposed to adapt a scatterometer of the type described above to perform coherent Fourier scatterometry. The aim of this modification is obtain phase information of the diffraction spectrum, as well as intensity information. The availability of phase information allows a more confident reconstruction. The method disclosed in US2012243004A1 requires multiple diffraction spectra to be captured and compared to obtain the phase information. Therefore it incurs a penalty in throughput, that is to say fewer measurements can be made in a given time. In a high-volume manufacturing environment, throughput as well as accuracy should be maximized.